Added fallback value for empty CONFIGURED_URL

If master-node.json does not contain "master-configured-url", then
CONFIGURED_URL in /etc/focus/local_settings.py contains empty string
and invitation email contains invitation link without protocol,
port, and domain name. This patch makes 'http://<hostname>/' default
value for this situation.

Change-Id: Ic9656094e43ee670b378a35ba44da3029d61134c